VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1938 truck gas gauge disassembled

I'm having trouble with the gas gauge on my truck. It reads correctly at empty (which I guess is the important end) because I drained only about 1.5 gallons from the tank after it hit empty. The problem is, when the tank is full, the gauge only reads 3/4 full.

I have done a 12 volt conversion and used a Runtz type voltage reducer on the gauge. I have checked the gauge itself by disconnecting the wire that attaches to the sender and the gauge goes to empty and grounded the wire and the gauge goes to full.

I removed the sender and made sure that everything moves freely.

The rheostat type strip inside the sender hat is what concerns me. I was under the impression that all early Chevy senders were 0 to 30 ohm. I used an adustable resistor in place of the sender and found that I have to provide about 48 ohms to make the gauge read full. The extreme end of the wire wound strip inside the sender hat only provides 42 ohms max.

Also, the factory service manual says it should be a 16 gallon tank, but it only took about 12 gallons to fill. Any ideas?

Heres a link to some pics of the disassembled sender.

http://community.webshots.com/album/569438939DAbzMY?start=36

Looks exactly like the one in my 38. If the sender reads 0 ohms in the empty position and 30 ohms (both + or - a bit) in the full position, it's working OK. I've read a lot of posts in various forums where folks swear you MUST put a resistor in line to the gas gauge when doing a 12v conversion. I converted my 55.1 panel to 12v and never even thought about the gas gauge needing special treatment. I put over 100K on that truck after the conversion and never had a bit of trouble with the gas gauge.

The gauge in my 37 will only go to 3/4 when full and when its a little above 1/4 start looking for a gas station,its still got the original sending unit and probably needs adjustment.

I got to thinking some more about this problem. I took the gauge out of the dashboard and found that if I grounded a certain terminal on top of the gauge itself it reads properly. It seems that over the years, extra resistance has built up inside the gauge itself and results in a bad ground i.e. there is extra resistance in the circuit where the gauge should go directly to ground causing the gauge to read full. Anyway, I broke one of the tiny, tiny wires in the gauge causing it to not work at all as I was putting it back together so now it is out for a total rebuild. So FYI to anyone experiencing the same problem, disassemble the gauge, ground the upper rightmost terminal when viewing the gauge from teh front and see if that doesn't fix the problem. Just be very carefull when reassembling.